A diesel engine is mounted on construction machines such as hydraulic shovel, bulldozer, damp truck, wheel loader and the like.
Describing the outline of the configuration of a conventional construction machine 1 using FIG. 1, a hydraulic pump 3 is driven with a diesel engine 2 as a drive source, as shown in FIG. 1. A variable displacement hydraulic pump is used for the hydraulic pump 3, where capacity q (cc/rev) is changed by changing a tilt angle etc. of a swash plate 3a. The pressurized fluid discharged from the hydraulic pump 3 at a discharge pressure PRP and flow rate Q (cc/min) is supplied to each hydraulic actuator 31 to 36 such as boom hydraulic cylinder 31 via operation valves 21 to 26. Each operation valve 21 to 26 is operated through operation of each operation lever 41, 42. When pressurized fluid is supplied to each hydraulic actuator 31 to 36, each hydraulic actuator 31 to 36 is driven, and a working machine including a boom, an arm, a bucket etc., a lower crawler carrier, and an upper rotation body connected to each hydraulic actuator 31 to 36 are operated. While the construction machine 1 is operating, the load applied on the working machine, the lower crawler carrier, and the upper rotation body continuously changes according to the excavating soil quality, traveling path gradient and the like. The load (hereinafter referred to as hydraulic equipment load) of the hydraulic equipment (hydraulic pump 3), that is, the load on the engine 2 accordingly changes.
The control of the output P ((horsepower) kw) of the diesel engine 2 is carried out by adjusting the fuel amount to be injected into the cylinder. This adjustment is performed by controlling a governor 4 arranged next to a fuel injection pump of the engine 1. Generally an all speed control type governor is used for the governor 4, and the engine revolutions and the fuel injection amount (torque T) are adjusted according to the load so that a target engine revolution set through fuel dial is maintained. That is, the governor 4 increases and decreases the fuel injection amount so that a difference between the target revolution and the engine revolution is eliminated.
FIG. 2 shows a torque curve diagram of the engine 1, where the horizontal axis is the engine revolution n (rpm: rev/min) and the vertical axis the torque T (N·m).
In FIG. 2, the region defined by a maximum torque curve R shows the performance the engine 2 can exhibit. The governor 4 controls the engine 2 so that the torque T does not become the exhaust smoke limit exceeding the maximum torque curve R and so that the engine revolution n does not become over rotation exceeding a high idle revolution nH. The output (horsepower) P of the engine 2 becomes a maximum at a rated point V on the maximum torque curve R. J indicates an equal-horsepower curve at where the horsepower absorbed by the hydraulic pump 3 becomes equal-horsepower.
When set to the maximum target revolution with the fuel dial, the governor 4 carries out speed governing on a maximum speed regulation line Fe connecting the rated point V and the high idle point nH.
As the load of the hydraulic pump 3 becomes greater, the matching point at where the output of the engine 2 and the pump absorption horsepower balances moves towards the rated point V side on the maximum speed regulation line Fe. When the matching point moves towards the rated point V side, the engine revolution n is gradually decreased and the engine revolution n becomes rated revolution at the rated point V.
Thus, problems in that the fuel consumption rate is large (bad) and the pump efficiency is low arise when performing the work with the engine revolution n fixed at a substantially constant high revolution. The fuel consumption rate (hereinafter referred to as fuel consumption) is the consumption amount of fuel per one hour and output 1 kW, and is one index of efficiency of the engine 2. The pump efficiency is the efficiency of the hydraulic pump 3 defined by capacity efficiency and torque efficiency.
In FIG. 2, M shows the equal fuel consumption curve. The fuel consumption becomes a minimum at M1, which is the valley part of the equal fuel consumption curve M, and the fuel consumption becomes greater towards the outer side from the fuel consumption minimum point M1.
As also apparent from FIG. 2, the regulation line Fe corresponds to a region where the fuel consumption is relatively large on the equal fuel consumption curve M. Thus, according to the conventional control method, the fuel consumption is large (bad), which is not desirable in engine efficiency.
In the case of the variable displacement hydraulic pump 3, it is generally known that the capacity efficiency and the torque efficiency are high and that the pump efficiency is high the larger the pump capacity q (swash plate tilt angle) at the same discharge pressure PRP.
As also apparent from the following equation (1), if the flow rate Q of the pressurized fluid discharged from the hydraulic pump 3 is the same, the pump capacity q can be increased by lowering the revolution n of the engine 2. Thus, the pump efficiency can be enhanced by speed-reducing the engine 2.Q=n·q  (1)
Therefore, the engine 2 is operated in a low-speed region where the revolution n is low to enhance the pump efficiency of the hydraulic pump 3.
However, as also apparent from FIG. 2, the regulation line Fe corresponds to a high rotation region of the engine 2. Thus, the conventional control method has a problem in that the pump efficiency is low.
If the engine 2 is operated on the regulation line, the engine revolution lowers at high load and might cause engine stall.
On the contrary to a control method of substantially fixing the engine revolution regardless of the load, a control method of changing the engine revolution according to the lever operation amount and the load is disclosed in Patent Document 1.
In Patent Document 1, a target engine operating line L0 passing through the fuel consumption minimum point is set, as shown in FIG. 2.
The required revolution of the hydraulic pump 3 is calculated based on the operation amount etc. of each operation levers 41, 42, 43, 44, and a first engine required revolution corresponding to the pump required revolution is calculated. The engine required horsepower is calculated based on the operation amount etc. of each operation levers 41, 42, 43, 44, and a second engine required revolution corresponding to the engine required horsepower is calculated. The second engine required revolution is calculated as an engine revolution on a target operating line L0 of FIG. 2. The engine revolution and the engine torque are controlled so that greater engine target revolution of the first or the second engine required revolution is obtained.
As shown in FIG. 2, the fuel consumption, the engine efficiency, and the pump efficiency are enhanced by controlling the revolution of the engine 2 along the target engine operating line L0. This is because even when outputting the same horsepower and obtaining the same requested flow rate, transition can be made from high rotation, low torque to low rotation, high torque, the pump capacity q becomes large, and operation is made at a point close to the fuel consumption minimum point M1 on the equal fuel consumption curve M when matched at the point on the same equal horsepower line J, the point pt2 being on the target engine operating line L0, than when matched at point pt1 on the regulation line Fe. The noise is enhanced by operating the engine 2 in the low rotation region, and engine friction, pump unload loss, and the like are enhanced.
In the field of construction machine, a construction machine of hybrid type that assists the driving force of the engine by the generator motor is being developed, and many have been applied for patent.
In Patent Document 2, the engine 2 is controlled along the regulation line Fe0 corresponding to the set revolution set with the fuel dial with reference again to FIG. 2. The target revolution nr corresponding to a point A at where the regulation line Fe0 and the target engine operating line L0 intersect is obtained, where the generator motor is electrically motor-operated to assist the driving force of the engine 2 with the torque generated by the generator motor if the deviation of the engine target revolution nr and the current engine revolution n is positive, and the generator motor is generator, operated to store power in an electrical storage device if the deviation is negative.    Patent Document 1: Japanese Patent Application Laid-Open No. 11-2144    Patent Document 2: Japanese Patent Application Laid-Open No. 2003-28071